[0001] This invention relates to a measurement method and apparatus therefor and, more particularly,
to a method and apparatus for measuring the coordinates of a point on an object, or
the distance to the object.
[0002] A method is available in which a measuring element is brought into contact with the
surface of an object to measure the coordinates of a point on the object surface or
the distance to the object. A method of measuring depth, for example, includes (a)
commanding an amount of movement in excess of actual depth, (b) generating distributed
pulses by performing a pulse distribution computation based on the commanded amount
of movement, (c) transporting a measuring element by driving a servomotor in response
to the distributed pulses, and (d) bringing the measuring element to rest after it
contacts the object, and measuring depth by relying upon the actual amount of movement
the measuring element made until coming to rest.
[0003] The servo system which drives the servomotor controls the motor in such a manner
that a difference (positional deviation) Er between the number of distributed pulses
and a number of pulses sensed each time the measuring element is moved by a prescribed
amount, will approach zero. Specifically, assume that the distributed pulses ordinarily
have a pulse rate of Fi, and that the gain of the servo system is k. In such case
the servomotor will be controlled so as to rotate with a delay equivalent to Er =
Fi/k under steady conditions, and so that the delay will approach zero. The delay
Fi/k (pulse number) is the difference between the number of distributed pulses and
the number of sensed pulses, and is stored in an error counter. When the distributed
pulses have ceased being generated, the servomotor will come to rest after rotating
by an amount corresponding to the aforesaid difference (i.e., positional deviation).
[0004] The foregoing leads to a problem in the conventional measuring method, which will
be understood from Fig. 1. As described above, the measuring element, represented
by ME, contacts the surface of the object being measured, designated by MW. Even though
the generation of distributed pulses is halted with such contact, however, the measuring
element will continue to travel a distance equivalent to Fi/k-number of pulses, and
will bite into the object as a result. This makes it impossible to obtain an accurate
measurement of position. An attempt at a solution has been to minimize the distributed
pulse rate (i.e., feed speed) Fi of the measuring element ME in order to reduce the
deviation Fi/k. However, a disadvantage with this expedient is that the lower pulse
rate prolongs the time needed for measurement.
[0005] Accordingly, an object of the present invention is to provide a measurement method
and an apparatus therefor through which the coordinates of an object or the distance
up to an object can be measured with great accuracy without prolonging measurement
time.
[0006] According to one aspect of the present invention there is provided a method of measuring
a coordinate value of a prescribed point on an object by moving a measuring element,
driven by a servomotor, toward said point and stopping the measuring element in dependence
upon its contact with the object at said point, comprising
(a) controlling the servomotor in such a manner that a difference between a number
of command pulses corresponding to a commanded amount of movement for moving the measuring
element toward the prescribed point on the object, and a number of feedback pulses
corresponding to an actual element, will aproach zero,
(b) producing an output signal when the measuring element contacts the object at the
prescribed point;
(c) halting the measuring element at the instant said output signal is generated;
the method being characterised by:
(d) computing at the instant said output signal is generated, the difference value
between said commanded amount of movement and a positional deviation, the latter being
equivalent to the difference between the number of command pulses issued up to said
instant, and the number of feedback pulses indicative of the amount of rotation of
the servomotor up to said instant, said difference value representing said coordinate
value.
[0007] According to another aspect of the present invention there is provided an apparatus
for measuring a coordinate value of a prescribed point on an object by moving a measuring
element, driven by a servomotor, toward said point and stopping the measuring element
in dependence upon its contact with the object at said point, said servomotor being
controlled by a servo system in such a manner that a difference between a number of
command pulses corresponding to a commanded amount of movement and a number of feedback
pulses corresponding to an actual amount of movement approaches zero, the apparatus
comprising:
pulse producing means which receives the commanded amount of movement as an input
for producing command pulses the number of which corresponds to said commanded amount
of movement;
commanded position counting means which receives the commanded pulses as an input
for producing an output signal indicative of a commanded position;
error counting means which receives the command pulses and feedback pulses indicative
of an amount of movement of the servomotor, for producing an output signal indicative
of a positional deviation equivalent to the difference between the number of command
pulses and the number of feedback pulses;
the apparatus being characterised by:
arithmetic means which receives the commanded position from said commanded position
counting means and the positional deviation from said error counting means for computing
the difference value between said commanded position and said positional deviation
at the instant that the measuring element contacts the object.
[0008] Features and advantages of an example of the present invention will be apparent from
the following description taken in conjunction with the accompanying drawings, in
which:
Fig. 1 is an explanatory view useful in describing a method of measurement according
to the prior art, and
Fig. 2 is a block diagram of an apparatus for practicing a method of measurement according
to the present invention.
[0009] Reference will now be had to the block diagram Fig. 2 to describe an apparatus for
practicing a measurement method according to the present invention. PDC designates
a pulse distributor for producing distributed pulses Zdp by performing a pulse distribution
computation on the basis of position command data Zc, namely a commanded amount of
movement. The distributed pulses Zdp are applied to an error counter ERC, which also
receives an input which is generated whenever a servomotor SM for Z-axis drive rotates
by a predetermined amount. The error counter ERC has its status counted up or down
by the pulses Zdp, Fp, depending upon the direction of movement, thereby recording
the difference Er between the number of commanded pulses (distributed pulses) and
the number of feedback pulses. This difference is referred to as a positional deviation.
More specifically, when the commanded direction is forward, the error counter ERC
has its status counted up each time a command pulse (distributed pulse) Zdp is generated.
When the commanded direction is the reverse direction, the counter status is counted
down by each command pulse Zdp. On the other hand, the error counter ERC is counted
down by the feedback pulses Fp when the indicated direction of movement is forward,
and is counted up by these pulses for movement in the reverse direction. The output
Er of the error counter ERC is applied to a digital-to-analog converter DAC, which
responds by producing a signal Ve serving as an analog speed command the magnitude
whereof is proportional to the positional deviation Er. The servomotor SM mentioned
above is, e.g., a DC motor for transporting a measuring element along the Z axis.
A position sensor RE, such as a rotary encoder or resolver operatively associated
with the servomotor SM, produces the feedback pulses Fp, each pulse being generated
whenever the motor makes a predetermined amount of rotation. A tachometer TM is coupled
to the servomotor SM for producing a voltage Vs representing the actual speed of the
servomotor, namely a voltage proportional to the rotational speed thereof. An adder
ADD receives the analog speed command value Ve and the actual speed voltage Vs, which
is also an analog quantity, and computes a differential voltage Vi between Ve and
Vs. The differential voltage Vi is applied to a speed control circuit VCC which controls
the rotational speed of the servomotor SM in such a manner that Vi will be reduced
to zero.
[0010] Designated at PRE is a present position counter whose status is counted up or down,
in accordance with the direction of movement, whenever a distributed pulse Zdp is
generated by the pulse distributor PDC, the counter thereby recording a commanded
position Zs along the Z axis at all times. The present position counter PRE applies
Zs to a subtractor SBT, which also receives the positional deviation Er from the error
counter ERC. The subtractor SBT computes the difference between Zs and Er performing
the arithmetic operation:

thereby producing a signal Zp, which will indicate the Z-axis coordinate of the object
MW at the point contacted by the measuring element.
[0011] The operation of the apparatus shown in Fig. 2 will now be described in greater detail.
In performing measurement, a control unit (not shown) supplies the pulse distributor
PDC with the commanded amount of movement Zc for travel along the Z axis, and the
pulse distributor PDC responds by performing a pulse distribution computation to produce
the distributed pulses Zdp. These pulses are fed into the error counter ERC so that
the pulses will be recorded in accordance with the commanded direction of movement.
For example, if the direction is -Z, the status of the error counter ERC will be decremented
by one step each time a distributed pulse Zdp is generated. The digital status of
the counter ERC is converted into the speed command of magnitude Ve by the D-A converter
DAC, and the adder ADD computes the difference Vi between Ve and the analog voltage
Vs indicative of the actual rotational speed of the servomotor SM, the latter being
driven in accordance with Vi. As a result, the measuring element, not shown, is transported
along the -Z axis toward the object being measured. As the servomotor SM rotates,
the position sensor RE generates one feedback pulse Fp each time the motor rotates
through a prescribed angle. The feedback pulses Fp enter the error counter ERC and
count up its contents. This operation continues in the manner described until the
measuring element comes into contact with the surface of the object being measured.
[0012] When the measuring element contacts the object, a contact signal CTS is generated
and applied to the pulse distributor PDC, error counter ERC and subtractor SBT. In
response thereto, the pulse distributor PDC immediately ceases the pulse distribution
operation, the contents of the error register ERC are cleared to zero, thereby halting
the measuring element, and the subtractor SBT performs the operation of Eq. (1). In
the latter, Zs is supplied by the commanded position counter PRE, the status whereof
was updated by the distributed pulses Zdp in the positive or negative direction, depending
upon the direction of movement. Thus, by performing the operation of Eq. (1), the
subtractor SBT produces a value, namely Zp, which represents the Z-axis coordinate
of the object at the point thereon contacted by the measurng element.
[0013] In accordance with the present invention as described and illustrated hereinabove,
movement of the measuring element is halted, and the operation of Eq. (1) is performed,
as soon as the measuring element contacts the object being measured. As a result,
highly accurate measurement is possible without prolonging measurement time, the latter
because it is unnecessary to minimize the feed speed of the measuring element.
[0014] While the foregoing description deals with measurement of the coordinate of a point
on the surface of an object, the invention can also be applied to measure the distance
up to the point.
[0015] Further, according to the foregoing description, the operation of Eq. (1) is performed
in response to generation of the contact signal CTS. However, an arrangement is possible
wherein the measuring element is made spherical in shape (such as the stylus used
in a tracer apparatus), with the operation of Eq. (1) being performed when the magnitude
of a relative displacement vector between the object and measuring element exceeds
a predetermined value.
1. A method of measuring a coordinate value of a prescribed point on an object (MW)
by moving a measuring element (ME), driven by a servomotor (SM), toward said point
and stopping the measuring element (ME) in dependence upon its contact with the object
(MW) at said point, comprising
(a) controlling the servomotor (SM) in such a manner that a difference (Er) between
a number of command pulses (Zdp) corresponding to a commanded amount of movement (Zs)
for moving the measuring element (ME) toward the prescribed point on the object (MW),
and a number of feedback pulses (Fp) corresponding to an actual amount of movement
made by the measuring element (ME), will approach zero,
(b) producing an output signal (CTS) when the measuring element (ME) contacts the
object (MW) at the prescribed point;
(c) halting the measuring element (ME) at the instant said output signal (CTS) is
generated;
the method being characterised by:
(d) computing, at the instant said output signal (CTS) is generated, the difference
value (Zp) between said commanded amount of movement (Zs) and a positional deviation
(Er), the latter being equivalent to the difference between the number of command
pulses (Zdp) issued up to said instant, and the number of feedback pulses (Fp) indicative
of the amount of rotation of the servomotor (SM) up to said instant, said difference
value (Zp) representing said coordinate value.
2. The method according to claim 1, wherein the measuring element (ME) is spherical
in shape, and said output signal (CTS) is produced, to indicate the instant of contact,
when the magnitude of the relative displacement vector between the object (MW) and
the measuring element (ME) attains a predetermined value.
3. An apparatus for measuring a coordinate value of a prescribed point on an object
(MW) by moving a measuring element (ME), driven by a servomotor (SM), toward said
point and stopping the measuring element (ME) in dependence upon its contact with
the object (MW) at said point, said servomotor (SM) being controlled by a servosys-
tem in such a manner that a difference (Er) between a number of command pulses (Zdp
corresponding to a commanded amount of movement (Zs) and a number of feedback pulses
(Fp) corresponding to an actual amount of movement approaches zero, the apparatus
comprising:
pulse producing means (PDC) which receives the commanded amount of movement as an
input (Zc) for producing command pulses (Zdp) the number which corresponds to said
commanded amount of movement;
commanded position counting means (PRE) which receives the commanded pulses (Zdp)
as an input for producing an output signal (Zs) indicative of a commanded position;
error counting means (ERC) which receives the command pulses (Zdp) and feedback pulses
(Fp) indicative of an amount of movement of the servomotor (SM), for producing an
output signal (Er) indicative of a positional deviation equivalent to the difference
between the number of command pulses (Zdp) and the number of feedback pulses (Fp);
the apparatus being characterised by:
arithmetic means (SBT) which receives the commanded position (Zs) from said commanded
position counting means (PRE) and the positional deviation (Er) from said error counting
means (ERC) for computing the difference value (Zp) between said commanded position
and said positional deviation at the instant that the measuring element (ME) contacts
the object (MW).
1. Meßmethode zum Messen eines Koordinatenwerts eines vorgegebenen Punkts auf einem
Objekt (MW) durch Bewegen eines Meßelements (ME), das durch einen Servomotor (SM)
angetrieben wird, in Richtung auf den Punkt und Anhalten des Meßelements (ME) in Abhängigkeit
von dessen Berührung mit dem Objekt (MW) bei dem Punkt, mit
(a) einem Steuern des Servomotors (SM) in einer Weise, daß eine Differenz (Er) zwischen
einer Anzahl von Befehlsimpulsen (Zdp), die mit einem Betrag der Bewegung (Zs) zum
Bewegung des Meßelements (ME) in Richtung auf den vorgegebenen Punkt auf dem Objekt
(MW) korrespondiert, und einer Anzahl von Rückkopplungsimpulsen (Fp), die mit einem
Istbetrag der Bewegung korrespondiert, welche durch das Meßelement (ME) ausgeführt
wird, gegen Null geht,
(b) einem Erzeugen eines Augangssignals (CTS), wenn das Meßelement (ME) das Objekt
(MW) bei dem vorgegebenen Punkt berührt, und
(c) einem Anhalten des Meßelements (ME) in dem Augenblick, zu dem das Ausgangssignal
(CTS) erzeugt wird,
gekennzeichnet durch
(d) ein Berechnen des Differenzwerts (Zp) einer Differenz zwischen dem befohlenen
Betrag der Bewegung (Zs) und einer positionsmäßigen Abweichung (Er) in dem Augenblick,
zu dem das Ausgangssignal (CTS) erzeugt wird, wobei letztere zu der Differenz zwischen
der Anzahl der Befehlsimpulse (Zdp), die bis zu diesem Augenblick ausgegeben wurden,
und der Anzahl der Rückkopplungsimpulse (Fp), die kennzeichnend für den Betrag der
Drehung des Servomotors (SM) bis zu diesem Augenblick ist, äquivalent ist und wobei
der Differenzwert (Zp) den Koordinatenwert respräsentiert.
2. Meßmethode nach Anspruch 1, bei der das Meßelement (ME) eine ballige Form hat und
das Ausgangssignal (CTS) erzeugt wird, um den Augenblick der Berührung zu kennzeichen,
wenn die Größe des Vektors der relativen Abweichung zwischen dem Objekt (MW) und dem
Meßelement (ME) einen vorbestimmten Wert erreicht.
3. Vorrichtung zum Messen eines Koordinatenwerts eines vorgegebenen Punkts auf einem
Objekt (MW) durch Bewegen eines Meßelements (ME), das durch einen Servomotor (SM)
angetrieben wird, in Richtung auf den Punkt und Anhalten des Meßelements (ME) in Abhängigkeit
von dessen Berührung mit dem Objekt (MW) bei dem Punkt, wobei der Servomotor (SM)
durch ein Servomotor derärt gesteuert wird, daß eine Differenz (Er) zwischen einer
Anzahl von Befehlsimpulsen (Zdp), die mit einem Betrag der Bewegung (Zs) zum Bewegung
des Meßelements (ME) in Richtung auf den vorgegebenen Punkt auf dem Objekt (MW) korrespondiert,
und einer Anzahl von Rückkopplungsimpulsen (Fp), die einem Istbetrag der Bewegung
korrespondiert, welche durch das Meßelement (ME) ausgeführt wird, gegen Null geht,
welche Vorrichtung
ein Impulserzeugungsmittel (PDC), das den befohlenen Betrag der Bewegung als ein Eingangssignal
(Zc) zum Erzeugen von Befehlsimpulsen (Zdp) empfängt, wobei die Anzahl dieser Befehlsimpulse
mit dem befohlenen Betrag der Bewegung korrespondiert,
ein Zählmittel (PRE) für die befohlene Position, das die befohlenen Impulse (Zdp)
als ein Eingangssignal zum Erzeugen eines Ausgangssignals (Zs) empfängt, welches für
eine befohlene Position kennzeichnend ist, sowie
ein Fehlerzählmittel (ERC), das die befohlenen Impulse (Zdp) und Rückkopplungsimpulse
(Fp), die für den Betrag der Bewegung des Servomotors (SM) kennzeichnend sind, zum
Erzeugen eines Ausgangssignals (Er) empfängt, welches für eine positionsmäßige Abweichung
kennzeichnend ist, die zu der Differenz zwischen der Anzahl von befohlenen Impulsen
(Zdp) und der Anzahl von Rückkopplungsimpulsen (Fp) äquivalent ist, enthält,
gekennzeichnet durch
ein arithmetisches Mittel (SBT), das die befohlene Position (Zs) aus dem Zählmittel
(PRE) für die befohlene Position und die positionsmäßige Abweichung (Er) aus dem Fehlerzählmittel
(ERC) zum Berechnen des Differenzwerts (Zp) zwischen der befohlenen Position und der
positionsmäßigen Abweichung in dem Augenblick, zu dem das Meßelement (ME) das Objekt
(MW) berührt, empfängt.
1. Un procédé de mesure d'une valeur de coordonnée d'un point prescrit sur un objet
(MW) en déplaçant un élément de mesure (ME), entraîné par un servomoteur (SM), vers
ledit point et en arrêtant l'élément de mesure (ME) en fonction de son contact avec
l'objet (MW) au niveau dudit point, comprenant,
(a) la commande du servomoteur (SM) de telle manière qu'une différence (Er) entre
un nombre d'impulsions de commande (Zdp) correspondant à une valeur commandée de mouvement
(Zs) pour déplacer l'élément de mesure (ME) vers le point prescrit sur l'objet (MW),
et un nombre d'impulsions de retour (Fp) correspondant à une valeur effective de mouvement
fait par l'élément de mesure (ME), va approcher de zéro;
(b) la production d'un signal de sortie (CTS) lorsque l'élément de mesure (ME) contacte
l'objet (MW) au niveau du point prescrit;
(c) l'arrêt de l'élément de mesure (ME) au moment où ledit signal de sortie (CTS)
est engendré;
le procédé étant caractérisé par:
(d) le calcul, au moment où ledit signal de sortie (CTS) est engendré, de la valeur
de différence (Zp) entre ladite valeur commandée de mouvement (Zs) et une déviation
de position (Er), cette dernière étant équivalente à la différence entre le nombre
d'impulsions de commande (Zdp) émises jusqu'audit moment, et le nombre d'impulsions
de retour (Fp) indiquant la valeur de rotation du servomotor (SM) jusqu'audit moment,
ladite valeur de différence (Zp) représentant ladite valeur de coordonnée.
2. Le procédé selon la revendication 1, dans lequel l'élément de mesure (ME) est de
forme sphérique, et ledit signal de sortie (CTS) est produit, pour indiquer le moment
de contact quand la grandeur du vecteur de déplacement relatif entre l'objet (MW)
et l'élément de mesure (ME) atteint une valeur prédéterminée.
3. Un appareil pour mesurer une valeur de coordonnée d'un point prescrit sur un objet
(MW) en déplaçant un élément de mesure (ME), entraîné par un servomoteur (SM), vers
ledit point et en arrêtant l'élément de mesure (ME) en fonction de son contact avec
l'objet (MW) au niveau duti point, ledit servomoteur (SM) étant commandé par un système
asservi de telle manière qu'une différence (Er) entre un nombre d'impulsions de commande
(Zdp) correspondant à une valeur commandée de mouvement (Zs) et un nombre d'impulsions
de retour (Fp) correspondant à une valeur effective de mouvement approche de zéro,
l'appareil comprenant:
des moyens de production d'impulsions (PDC) qui reçoivent la valeur commandée de mouvement
comme une entrée (Zc) pour produire des impulsions de commande (Zdp) dont le nombre
correspond à ladite valeur commandée de mouvement;
des moyens de calcul de la position commandée (PRE) qui reçoivent les impulsions commandée
(Zdp) comme une entrée pour produire un signal de sortie (Zs) indicateur d'une position
commandée;
des moyens de calcul d'erreur (ERC) qui reçoivent les impulsions de commande (Zdp)
et les impulsions de retour (Fp) indiquant une valeur de mouvement du servomoteur
(SM), pour produire un signal de sortie (Er) indicateur d'une déviation de position
équivalente à la différence entre le nombre d'impulsions de commande (Zdp) et le nombre
d'impulsions de retour (Fp);
l'appareil étant caractérisé par:
des moyens arithmétiques (SBT) qui reçoivent la position commandée (Zs) desdits moyens
de calcul de la position commandée (PRE) et la déviation de position (Er) desdits
moyens de calcul d'erreur (ERC) pour calculer la valeur de différence (Zp) entre ladite
position commandée et ladite déviation de position au moment où l'élément de mesure
(ME) contacte l'objet (MW).